Non-CO2 greenhouse gases: the underrepresented, complex side of the climate challenge

When it comes to mitigating climate change, the focus tends towards carbon dioxide (CO2) emissions. However, human-induced emissions of non-CO2 greenhouse gases (GHGs) are important contributors to global warming. These include methane (CH4), nitrous oxide (N2O), fluorinated greenhouse gases (HFCs, PFCs and SF6) and ozone depleting substances. Other non-CO2 emissions, such as aerosols (e.g. black carbon (BC)) and pollutants that function as aerosol or GHG precursors, contribute to changes in the atmosphere’s energy balance. This editorial overview introduces a Special Issue (SI) of Journal of Integrative Environmental Sciences that includes contributions from the 8 Non-CO2 Greenhouse Gas Symposium (NCGG8), which took place in Amsterdam, June 10–15, 2019. NCGG8 covered recent scientific advancements in understanding non-CO2 sources, climate implications and mitigation challenges. The articles in this special issue highlight some of outcomes of these discussions.


Niche topic, despite massive implications
Since the early 1990s, eight NCGG symposium editions have been organized in The Netherlands. The NCGG symposia bring together scientists and engineers on the one hand, and decision makers in the public and private sectors on the other hand. As such, these actively support the implementation of promising policies and technologies to reduce non-CO 2 GHG-emissions. Special attention is paid to environmental targets such as the United Framework Convention on Climate Change (UNFCC), and to the UN Sustainable Development Goals (SDGs). Based on preceding editions and particularly NCGG7 (Kroeze et al. 2014;Kroeze and Pulles 2015), it was concluded that there is a widely recognized essential role of non-CO 2 mitigation in reaching deep mitigation targets (limiting global warming to 2 degrees Celsius or lower), in line with recent literature (Frank et al. 2018;Rogelj et al. 2019;Harmsen 2019). However, it was found that this is not reflected in the attention for non-CO 2 related research and policy, compared to that of CO 2 . Moreover, the presented research has shown that a large share of non-CO 2 mitigation can be realized at relatively low costs. Interest in the topic has been increasing over the past decades, but it can still relatively be considered a niche. About 300 people attended NCGG8, comprising a large share of the leading non-CO 2 specialists. This is a small group compared to other research fields (e.g. picture approximately 10,000+ attendee medical symposia), considering the large implications, complexities and uncertainties that could be reduced by upscaling research. gases, in general, have a large diversity in sources and consequently in solutions. Clearly, there are countless possible solutions, but it is unclear what (combination) is most effective. The diversity in the Special Issue studies illustrates the complexity.
Many contributions during NCGG8 addressed long-term aim to limit global warming to 1.5 degrees Celsius. To meet this aim, net global GHG emissions need to be reduced to zero or lower within a generation. One of the observations during NCGG8 was that for several non-CO 2 greenhouse gases, it is technically possible to reduce emissions to very low levels. This holds in particular for emissions from energy and industry. However, it was also observed that it will be difficult to reduce total non-CO 2 GHG emissions to zero. The two sources that are most difficult to reduce are N 2 O emissions associated with food production, and CH 4 emissions from ruminants. This implies that in a 1.5 degree world, food production needs to be much more fertilizer efficient than today, but also that diets need to shift away from animal products, including both meat and milk. As earlier editions, NCGG8 focused on emission measurements (e.g. inverse modelling of emissions), as well as on understanding what drives these emissions locally (as reflected in this Special Issues' topical papers) and how these can be mitigated. We observe an increasing understanding of emission sources (although there are still mismatches between bottomup and top-down data) and a trend towards a more solution-driven policy approach in recent years. However, based on the large range of projected emission reduction potentials in the presented work, it can be concluded that large uncertainties remain, and casestudy-based assessments of promising mitigation measures are often lacking.
This Special Issue contains a selection of contributions to the symposium. It consists of this overview and seven topical papers as illustrative examples of the scientific results presented at the symposium (Wilson and Scholes 2020;O'neill et al. 2020;Godwin and Ferenchiak 2020;Howarth 2020;Laban et al. 2020;Verchot et al. 2020;Zou and Osborne 2020). In the following section, we give an impression of the seven contributions.

Topical papers in the special issue
The Special Issue papers described below are structured as follows. The first five mainly cover analyses of emission sources and determinants that influence emission fluxes (i.e., related to the 1st and 2nd theme of NCGG8). Combined, these papers cover a range of geographical settings and topics of land-use change (LUC). The last two papers are focused on policy, mitigation measures and emission accounting (i.e. more related to the 3rd and 4th theme of NCGG8) Laban et al. (2020) determined statistical relationships between surface ozone (O 3 ), its precursor species and meteorological conditions in South Africa. Multiple linear regression analysis revealed that atmospheric carbon monoxide (CO), temperature and relative humidity were the strongest factors affecting daily O 3 variability. Based on partial residual plots, it was found that temperature, radiation and nitrogen oxides most likely have a non-linear relationship with O 3 , while the relationship with relative humidity and CO is probably linear. In the winter, O 3 variability was more strongly associated with temperature and CO while in the summer variability relative humidity was the strongest factor. The findings emphasize how both regional-scale O 3 precursors and meteorological conditions impact daily variances of regional O 3 levels.

Zou and Osborne (2020) No effect of warming and watering on soil nitrous oxide fluxes in a temperate sitka spruce forest ecosystem
N 2 O soil fluxes influences the global greenhouse gas budget, but are less studied than CO 2 soil fluxes. Quantifying and understanding N 2 O soil fluxes are critical for anticipating potential nitrogen sinks and emissions. Zou and Osborne (2020) studied the Sitka spruce, an important tree type in Ireland's reforestation efforts, to understand nitrogen fluxes in relation to different levels of water stress and atmospheric warming. However, their findings suggest that not all forests beds are affected the same by climate change. In contrast with Griffis et al. (2017)'s study that proposed that drier and/or warmer soil compositions influences emissions, Zou and Osborne (2020) results from Dooaray Forest, central Ireland, indicate that soil N 2 O emissions in temperate forest ecosystems are not significantly affected. They conclude that more extreme temperature changes may be necessary in order to significantly influence increases to nitrogen emissions.

O'neill et al. (2020) Assessment of nitrous oxide emission factors for arable and grassland ecosystems
Many GHG emission estimates use default values to derive N 2 O emissions from fertilizer application in agriculture. However, O'neill et al. (2020) argue that these emission factor estimates need to be disaggregated and they challenge the use of emission calculations based on assumption that N 2 O emissions are linearly related to the nitrogen fertilizer rate. To this end, O'neill et al. (2020) studied to what extent N 2 O fluxes were affected by different fertilizing practices and land types (arable lands and grasslands) in south-east Ireland. For arable lands, they studied different tillage practices combined with varying fertilizer rates. For grasslands, they looked at mineral nitrogen, different rates of livestock slurry, and no fertilizer application. The general trend was that increased nitrogen fertilizer applications resulted in higher N 2 O fluxes. However, higher temperatures and higher soil moisture levels were linked to higher N 2 O production. Moreover, fertilizer practices in grasslands are related to higher variability in emissions than in arable lands. This indicates that measurements to understand fertilizer's contribution to N 2 O emissions, more citespecific is required.

Verchot et al. (2020) Land-use change and biogeochemical controls of soil CO2, N2O and CH4 fluxes in Cameroonian forest landscapes
LUC, and deforestation in particular, alters N 2 Osoil emissions. Tropical forests are also an important sink of atmospheric CH 4 . At the same time, most conversion of forests to agriculture occurs in the tropics and this affects both CH 4 and N 2 O contributions to total GHGs. However, uncertainty about how CH 4 and N 2 O fluxes are affected by LUC in tropical forests is high. Verchot et al.'s (2020) study looks at the data-limited region of Central Africa and zooms into how LUC and deforestation in the Congo affect soil-iv EDITORIAL atmosphere exchange. In addition to quantifying CO 2 , the study focuses on the influences of CH 4 and N 2 O. Soil gas fluxes were quantified for three LUCs: secondary forest, cocoa agroforest, and unfertilized cropland. They also studied biogeochemical mechanisms that explain temporal and spatial variation of these fluxes. Two key findings suggest that in LUC in tropical regions decreases soil respiration and the strength of CH 4 soil storage. Verchot et al. (2020) observed that in the absence of fertilizer use, N 2 O emissions decreased with LUC. They also found that soil water content, instead of temperature, was the dominant driver of variability; average fluxes in the wet season were more pronounced than in the dry season. Land-use type also affected the range of variability. Namely, unfertilized cropland's wet season had a 17% higher flux than its dry season whereas the seasonal fluxes for the other two land-use types were more pronounced, approximately 50% more variability. Thus, land type and soil content are important drivers of these fluxes in this region.

Wilson and Scholes (2020) The climate impact of land use change in the miombo region of south central Africa
Sustainable trajectories for land-use change (LUC) is critical for us to meet the 1.5 C target. However, incorporating non-CO 2 GHGs in LUC climate studies can lead to paradoxical results and different policy recommendations. Wilson and Scholes (2020) study contributes to the debate on LUC directions in Africa. They explore the implications of LUC on GHG emissions in the miombo region, which are woodlands located in south central Africa. These woodlands are vast and relatively unexploited, but have potential as agricultural lands. Wilson and Scholes (2020) analyse three patterns of LUC: low-input, lowyield smallholder subsistence farming; large-scale, high-input (pesticides, fertilization, and mechanization), high-yield commercial farming of commodity crops for export; and an improved high-yield, high-input smallholder farming. The last option attempts to minimize the negative implications of the first two methods by balancing the local needs of the community with goals of increased production and the environment by limiting deforestation. Surprisingly, they find that intensive commercial agriculture is the better than low-input (other than labour) traditional low-yield smallholder agriculture for climate goals based on surface reflexivity and net emission changes over 33 years of simulated time. Their model suggests that net brightening of the land surface going from dark forests to brighter/more radiant crop coverage has a net larger cooling effect than the loss of forested areas. They conclude that leaning too much on carbon dioxide emissions to assess LUC can result in misguided recommendations and advise integrating all of the major greenhouse gases in these assessments.

Howarth (2020) Methane emissions from fossil fuels: exploring recent changes in greenhouse-gas reporting requirements for the State of New York
Howarth (2020) provides an assessment of newly implemented greenhouse gas reporting rules in New York state. Two key additions in this calculation are the inclusion of out-ofstate emissions associated with in-state energy use and the shortening of the time frame from 100 years to 20. The inclusion of out-of-state emission helps reflect the emissions burden of transitioning from fuels such as coal to natural gas. The shorter time frame emphasizes the role of methane on short-term climate change effects. This new calculation paints a different picture of the current trajectory of New York State's GHG emissions. Traditional calculations of New York State's emissions indicated a 15% decrease in carbon emissions and thus, lower total GHG emissions, while the newer calculation suggest that total emissions have remained relatively unchanged with increased methane emissions replacing decreased carbon emissions.

Godwin and Ferenchiak (2020) The implications of residential air conditioning refrigerant choice on future hydrofluorocarbon consumption in the United States
Another paper that reflects the challenges of transitioning one material source to another is the paper by Godwin and Ferenchiak (2020). Hydrofluorocarbons (HFCs) have been used in air conditioning to replace ozone-depleting substances such as chlorodifluoromethane, which were phased out under the Montreal Protocol. However, HFCs also contribute to GHGs, and the World Meteorological Organization (WMO 2018) estimates limiting their use globally can mitigate 0.2-0.4 degrees Celsius temperature rise by 2100. Godwin and Ferenchiak (2020) explore the GHG implications of increased use of residential air conditioning (RAC) in the United States, which is expected to increase globally (UNEP 2019). Godwin and Ferenchiak (2020) model's findings suggest that improving the efficiency and use of virgin HFCs is insufficient to meet many possible emission targets alone. The long lifetime of RACs (5-40 years) coupled with the long time for manufacturing to convert to efficient RACs, limits emission reduction potential. They suggest that industry support is needed to limit leaks, recover and re-use the HFCs. RACs are an example of needing to integrate the supply chain to reduce the need for virgin HFC refrigerants and reduce inefficient HFC consumption.

Conclusions and discussion
The papers in this Special Issue help to address the gap of non-CO 2 GHG research identified in Kroeze et al. (2014) and Kroeze and Pulles (2015). Based on the outcome of NCGG8 conference, it can be concluded that non-CO 2 GHG related research remains an important, yet relatively small field, given the fact that 1) stringent climate targets require non-CO 2 mitigation to become a key part of global climate policy (Frank et al. 2018;Rogelj et al. 2019) and 2) non-CO 2 related policy will need to consist of a combination of many independent solutions, even more than in the case of CO 2 emissions. The topics addressed at NCGG8 and in this Special Issue seem to indicate a trend towards aiming to understand what drives (local) emissions and what implications this could have for mitigation options. Unsurprisingly, recent studies also show a stronger focus on assessing the role of non-CO 2 GHGs limiting global temperature increase by 1.5 degree Celsius, i.e. achieving the Paris Agreement's goals.
The Special Issue's topical papers do not fully cover all non-CO 2 GHG emission and mitigation-related topics (e.g. topics addressed at the symposium, but not or partly covered in the Special Issue are measurement techniques, mitigation measure assessments, policy assessment and global emission projections). However, the studies in the Special Issue address several key topics, such as emission sources and their determinants, atmospheric chemistry, climate impacts, mitigation options, scenario analysis and emission accounting and reporting. The Special Issue papers demonstrate some of the debates in emission studies and their diversity illustrates the complexity of the topic.

Disclosure statement
No potential conflict of interest was reported by the author(s).